1. Field of the Invention
The present invention relates to low NOx producing burner apparatus and methods, and more particularly, to low NOx axial premix burner apparatus and methods.
2. Description of the Prior Art
Because of stringent environmental emission standards adopted by government authorities and agencies, burner apparatus and methods have heretofore been developed which suppress the formation of nitrogen oxides (NOx) in flue gases produced by the combustion of fuel-air mixtures. For example, burner apparatus and methods wherein liquid or gaseous fuel is burned in less than a stoichiometric concentration of air to lower the flame temperature and thereby reduce thermal NOx have been developed. That is, staged air burner apparatus and methods have been developed wherein the fuel is burned in a deficiency of air in a first combustion zone whereby a reducing environment which suppresses NOx formation is produced, and the remaining portion of the air is introduced into a second zone downstream from the first zone wherein the unburned remaining fuel is combusted.
Staged fuel burner apparatus have also been developed wherein all of the air and some of the fuel is burned in a first zone with the remaining fuel being burned in a second downstream zone. In such staged fuel burner apparatus and methods, an excess of air in the first zone functions as a diluent which lowers the temperature of the burning gases and thereby reduces the formation of NOx.
While staged fuel burners which produce flue gases containing low levels of NOx have been utilized heretofore, there are continuing needs for improved axial premix burner apparatus having high firing capacities and producing flue gases having ultra low NOx emission levels and methods of using the apparatus.
By the present invention low NOx axial premix burner apparatus and methods are provided which meet the needs described above and overcome the deficiencies of the prior art. That is, in accordance with the present invention, a low NOx forming premix burner apparatus for burning gaseous fuels adapted to be connected to a furnace space is provided. The burner apparatus includes a housing having a discharge end attached to the furnace space and a closed opposite end. Means for introducing air into the housing are attached thereto and a burner tile having an opening therethrough and optionally including a flame stabilizing block as a part thereof is disposed within the furnace space adjacent to the burner housing. At least one elongated primary fuel gas and air venturi mixer is disposed within the housing having an open inlet end positioned adjacent to the closed end of the housing and a primary fuel gas-air mixture discharge nozzle attached to the other end thereof. The discharge nozzle extends into the burner tile through the opening therein and is positioned so that the flame produced by the burning of the primary fuel gas-air mixture is projected in a direction which is axial to the burner housing and impinges on the flame stabilizing block when it is utilized. A first primary fuel gas nozzle connected to a source of pressurized fuel gas is positioned to discharge a primary gas fuel jet into the open inlet end of the elongated venturi mixer whereby air from within the housing is drawn into the mixer, the air is mixed with the primary fuel gas therein and the resulting primary fuel gas-air mixture is discharged by the discharge nozzle and burned in the burner tile and in the furnace space. A second primary fuel gas nozzle connected to a source of pressurized fuel gas is positioned within the burner tile to discharge additional primary fuel gas into the flame therein whereby the flame is further stabilized. At least one secondary fuel gas nozzle connected to a source of pressurized fuel and positioned to discharge secondary fuel gas within the furnace space is provided whereby the secondary fuel gas mixes with air and flue gases in the furnace space and is burned therein.
The methods of the present invention basically comprise the following steps. (a) mixing a first portion of the fuel gas and all of the air to form a lean primary fuel gas-air mixture; (b) discharging the lean primary fuel gas-air mixture into a furnace space whereby the mixture is burned in a primary combustion zone therein and flue gases having very low NOx content are formed therefrom; (c) discharging a second portion of the fuel gas into the primary combustion zone whereby the second portion of the fuel gas is mixed with air and is burned to further stabilize the flame produced therein; and (d) discharging the remaining portion of the fuel gas into a secondary combustion zone in the furnace space wherein the remaining portion of the fuel gas mixes with air in the furnace space and with flue gases therein to form a second fuel gas-air mixture diluted with flue gases whereby the mixture is burned in the secondary combustion zone and additional flue gases having very low NOx content are formed therefrom. The flame produced in the primary combustion zone by the burning of the lean primary fuel gas-air mixture discharged in accordance with step (a) can optionally contact a flame stabilizing block in the furnace space.
It is, therefore, a general object of the present invention to provide an improved low NOx axial premix burner apparatus and methods of burning an at least substantially stoichiometric mixture of fuel gas and air whereby flue gases having very low NOx content are formed therefrom.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows when taken in conjunction with the accompanying drawings.